The shortest answer to this question is you should bid higher, a lot higher for your call-only campaign.
We are going calculate how much we need to bid for call-only ads. Let us assume that the text search campaigns are running at 10$ per conversion and the average conversion ratio is 10%. Then your text search campaigns are giving you clicks at 1$ each. We would recommend you to bid 10 times higher for your call-only campaign ads.
In our strategy, we assume that a call to your phone is as valuable as a lead that is generated by other campaigns. If this is not the case you might have to adjust a bit on bidding.
A simple formula to calculate the bids for a call-only campaign is to multiply it with the inverse of conversion ratio of text campaigns. For Eg: if the conversion ratio is 5%, multiply the bids by 20. If the conversion ratio is 50%, multiply by 2.
Call-Only Campaign bid = Text-Only Campaign Bid/Conversion Ratio
The basics and the strategy for call-only campaigns creation are explained in depth for the benefit of you. It is very important to set up effective campaigns. This is because a call from a prospect is a step much closer than a click.