It actually isnt a bad idea having your line with your ISP. My adsl was down, so i called WebAfrica and reported the fault. The agent mentioned to me that a coworker was waiting a while for a telkom agent to answer but assured me that my number will be added to the batch of people experiencing problems with their line. The problem was reported and my line was restored within 2 hours. It saved me the hassle of dealing with Telkom.
I think you should only hand your line over to an ISP if:
*they have provided consistent service
*they are reliable
*you are satisfied with the level of customer service.
Many people on mybb complain about the support from WebAfrica but i have yet to have any issues with them. I always phone them and never use the online chat service.
I have the impression that Crystalweb will go out of their way to solve your issue and maybe mweb will be helpful.
I will however never transfer my line to Afrihost. I trust Afrihost as much as i trust Zuma. Afrihost should market itself as a PR company because they are always trying to do damage control on mybb.