Calling Garage with SIP voip.

I'm currently using sip voip from my smartphone to call garages, because I don't want my company to see my calls to the garages and don't want to swap sims.  I've had mixed reviews, b/c the garages act all weird when CID doesn't come thru on their end.  Their typically friend customer service becomes all weird and they keep saying....we are all booked today...thank you. Then hang up. Any c-hing experience this? Is the only solution is to get another prepaid sim?