Once a week I get a call about a toilet that is not flushing. I noticed that there is bubbles that appear in the bowl. When I flush the toilet, what could be the problem. Alfonso

Likely a clogged vent stack, rod from the roof. The bubbles are the air inside the drain which can't vent via the stack

Right next to the toilet is a sink, when I flush the sink under pressure it eventually unclogs the toilet. Then the toilet will flush for several days, until I get another cal on it, is this still the vent pipe.

Could be, they share the same vent. Before going onto the roof this time of year why not try rodding down the sink drain with the trap pulled.

Will try thanks a bunch. Alfonso

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I noticed that there is bubbles that appear in the bowl.

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I* have a different take on your problem. Let me explain about bubbles and gurgles.
When your fixtures began to talk back to you they do it in two ways. They "gurgle" atcha or they "bubble" atcha. The "gurgle" is caused by vacuum caused by a blocked vent. The noise you hear is the sound of the air venting through the water of the trap. The "bubble" is caused by back pressure due to a partial blockage What happens is that when you flush or drain the discharge runs down the drain line and hits the partial blockage and bounces back sending a bubble of air ahead of it. A partial blockage will also cause a backup sending soap or crud into a fixture until it can drain away.
A septic tank that, (if filled up because the drain field is no long able to handle the discharge from a fixture that dispenses so much volume in short a period that it loads the tank up), needs to be pumped.would give you bubbles in the toilet bowl.
I would check both the line from the house to the septic tank for a partial clog and the tank itself to see if it needs to be pumped. If the tank's full and should be pumped I would also want the drain field checked to see if I had a problem there.
In short In don't think you have a vent problem. I think you have a partial blockage downstream from the toilet. If you're wet vented this could be snaked from the lavatory roof vent, (Put out enough snake to reach the base and 20 feet more), OR you could also pull the toilet and snake from there. Good luck, Tom