Where should a wireless desk phone be placed in relationship to modem or computer? Or should a conventional telephone be used near computer or modem?

If you have wireless network, 2.4 Ghz and some cordless phones work in the 2.4
Gig range. Would keep separate.
Personally I would use Landline, Cordless may compromise security, if with tech support
And you have to give important info, Nearby scanner can pick up. Credid Card info etc...
You could have the best security, but speaking over the air makes it vurnerable.
Hardwire WILL NOT create problem.