Looking for SIP Trunk provider to resell to SMBs

I am starting to offer IP PBXs using FreePBX and I’m looking for a good, inexpensive SIP Trunk provider to resell or just to suggest to clients. Probably less than 25 trunks/DIDs per client, but wanting a company that is easy to work with for porting numbers. I also doubt the volume of clients will be that large(at least at start). Any suggestions?