Telkom Mobile doesn't roam on Vodacom network

I have a Telkom sim, frequently I do not get signal when Vodacom sims do. I don't believe that roaming on the Vodacom network is working as promised. I frequently go through days of no service where there is signal but I get a 'not registered on network' fault when trying to call. I can't even get through to customer service as I reportedly should be able to do.