Is there anything wrong with telemarketers for a charity using pressure tactics to get people to donate?
R. Aaron Levine, Moral Issues of the Marketplace in Jewish Law, pp. 241-242:
A variation of the above case occurs when Arrow [the telemarketer] is a charity solicitor. R. Bezalel Stern dealt with an analogous case: A Rabbi (R) had in his possession matzah for the night of Passover in excess of his own religious requirement (mitzvah) needs. His friend (F) had no matzah to fulfill his mitzvah need and therefore requested R to either sell or give him the excess supply. Here, F’s efforts to overcome R’s initial resistance to make the matzah available to him violates neither the lo tit’avveh nor the lo tahmod interdicts. This is so because absent F’s pleadings and exertions, R, is, in any case, obligated to make the matzah available to F so that he can fulfill his mitzvah need. Similarly, in the case at hand, Oak [the person being called] is obligated to give charity as a religious duty. Making use of persuasion of all sorts to overcome Oak’s initial rejection to contribute to the charitable cause should therefore not entail a violation of lo tit’avveh and lo tahmod on the part of Arrow. Similarly, Arrow’s persistence and persuasion to overcome Oak’s resistance should not amount to causing him needless mental anguish and should therefore not violate ona’at devarim. One caveat, however. To be sure, Oak has a religious duty to give charity. But, Oak may feel that he is not obligated to support the particular organization Arrow is soliciting funds for. Accordingly, Arrow does not have an unlimited license to push his cause on Oak. At some point, Arrow’s persistence and pestering become a violation of ona’at devarim.
 R. Bezalel Stern, Be-zel ha-Hokhmah 3:43.
 Leviticus 25:35; Deuteronomy 15 7–8, 10. In respect to agricultural produce, the Torah prescribes a ten percent obligation (Deuteronomy 14:22). Talmudic decisors differ as to whether the ten percent benchmark applies to income as well. Opinions in the matter range from an income tithe requirement arising from biblical law to one established by rabbinical edict. In his survey of the Responsa literature, R. Ezra Basri concludes that the majority opinion regards the ten percent level as a definite obligation, albeit by dint of rabbinical decree. In any case, devoting less than ten percent of one’s income to charity is considered by the rabbis to reflect an ungenerous nature. (R. Ezra Basri, Dinei Mamonot vol. 1, p. 405).