Using the telephone should not impact your internet or catch-up TV. Voice down the phone line and data travel down the same line but at different frequencies, so they should not interfere. I say "should" as it's not impossible to have a fault where they do interfere, usually a fault with the microfilter, but that's not very common.
TV boosts won't be impacted by other internet usage, but they will impact other internet users Boosts get priority and take about 2.5Mb of bandwidth away from other internet users in the house. Whether anyone would notice that depends on your total bandwidth. If you only have 5Mb download, others may notice it.
Normal catch-up (iPlayer etc) fights for bandwidth with other internet users. It typically needs 2Mb to avoid buffering. Someone else surfing / emailing won't cause a problem. Others downloading heavily / watching video, may cause on-demand to buffer, depending on your total bandwidth. If you have 5Mb bandwidth and you're trying to watch catch-up while two other people are on Netflix, then on demand will struggle.
Running a speed test to get some idea of your download bandwidth would help tell if lack of bandwidth could be causing your on-demand problems.