I'm pretty sure that at the start Tiswas was just a series of links between programmes rather than programme itself. The name 'Tiswas' wasn't used from the start either
Well, Yes and No!
Originally Peter Tomlinson (who I interviewed for TiswasOnline and the ATV documentary that we're doing) was doing continuity in ATV Midlands at the weekends in the early 1970s. To cut a long story short, he had some time to fill between cartoons and episodes of Tarzan so he had the idea of running some small competitions and the response was unexpectedly large. They carried on with this idea over a few weeks and realised that if organised properly it had great potential. This was during the middle of 1973. I believe, from Peter, that he may have included the odd phone call within this sequence (unsurprisingly there were no examples of this period ever recorded).
When the decision was taken to make the Saturday Morning sequence into a 'stand alone' programme, Peter Harris (also extensively interviewed for our project!) was assigned to the project as Producer. Having next to no budget, they made use of Studio 3 at ATV in Birmingham, grabbed a desk and a few chairs from one of the offices (sometimes making use of parts of the ATV Today set if it was still standing in the studio) and from Jan 1974 launched it as a 'stand alone' programme billed as "Today Is Saturday or the Tis-was Show" (the title sequence showed both a "Today Is Saturday" caption and a chunky lettered "Tiswas" logo").
The show grew organically from then on, but never really had much of a budget to speak of until the so-called 'classic' period of 1979-80 onwards.
I guess you could compare it to the way that Philip Schofield's original Children's BBC links grew directly into "But First This" during the school holidays.
It was never a major 'phone-in' type of show, but it did do it before Swap Shop!