State of California Ex Parte filing.
My husband and I made an agreement that I would take a very good paying job in San Diego while he remained in Northern Ca. with the children for a couple of weeks until they were out of school (ages 5 and 9) . The agreement was that they would then come to San Diego with me for the summer. He is now refusing to let me take the kids for the summer unless I sign some noterized doc stating that I will return the children to him for the school year. He had previously made all sorts of other demands of what I do before I can take the kids for the summer as per our agreement but this is the most current of his demands. We are separated and plan to divorce since he has a huge controlling issue and is a habitiual lier of which he was in counseling for. Additionally, the counseler said that he needed to be on anxiety meds but he refused.

When I refused to sign his form, he then filed an ex parte and says he can make this all go away if I just sign the form.There's not room here to go into all details but can I respond to the ex parte by phone since I am in another county? There is no emergency for him just more of his controlling

So who has court ordered custody ? If there is no court ordered custody, he has full rights to keep the kids.
If you have custody though the court take him back to court to force him to return the kids

In your response if the exparte motion is accepted for a hearing then you will have to ask to appear by telephonic means. But at this point if there is a custody issue and this is your first trip through the courts they will most likely recommend you have at least one session with a mediator before the hearing to see if the both of you can reach an agreement.

If anyone were to file exparte it would have been you to justify the visitation for the summer. Since currently he has the children in his possession.

An ex parte (http://en.wikipedia.org/wiki/Ex_parte) motion is, strictly speaking, one which can be made without notifying the other party before the order is made. OP appears to have received a copy of the motion so, In this case, evidently it is being used in the sense of the motion being made and decided without the necessity of either a hearing or a written response by the OP.

Therefore I don't know whether there will even be a hearing. Did you receive a notice of a hearing, sbmjellybean?

State of California Ex Parte filing.
My husband and I made an agreement that I would take a very good paying job in San Diego while he remained in Northern Ca. with the children for a couple of weeks until they were out of school (ages 5 and 9) . The agreement was that they would then come to San Diego with me for the summer.

Was this agreement through the courts? Is it in writing at least?

I agree with AK, who is filing a motion here? Since the children are currently in his custody, he doesn't have to do anything. If you want the children for the summer, then YOU have to file a motion with the courts. So something doesn't make sense here.

... he then filed an ex parte and ... can I respond to the ex parte by phone ...

The words "ex parte" constitute an adjective which would modify the noun "motion". S o when you used that phrase, I was assuming that's what you meant. The phrase "Ex parte" is not a noun phrase, and therefore does not really make sense by itself.